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Point of aim shift question.


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  • Point of aim shift question.

    I just checked my zero after free floating the barrel on my old 788 .308. I'd say floating the barrel was a success. At the risk of bragging up my rifle the first three shot group out of her measured .319. Windage was dead center but one inch high. My question is when I fired my second group they printed three quarters left. The target was twelve inches left of the first. I fired the second group from the same position as the first. not adjusting my body position at all just moving the rear bag to the right. Would not shifting my body even for this small difference in target location cause the group shifting left?

  • #2
    You pretty much answered your own question. The answer is yes but you still have to determine which position was your natural point of aim.


    • #3
      That was my question. I would assume the first group I was directly behind the rifle. Firing the second group I did not shift my body to the right just the bag. So I was not behind the rifle as well. Would a foot at 100 yards cause the group to print left because I did not shift my hips to the right?


      • #4
        Originally posted by Goosedog1 View Post
        That was my question. I would assume the first group I was directly behind the rifle. Firing the second group I did not shift my body to the right just the bag. So I was not behind the rifle as well. Would a foot at 100 yards cause the group to print left because I did not shift my hips to the right?
        First, I may be assuming but, point of aim (POA) should have stayed the same and your true intent here is the shift in point of impact (POI). Correct?
        Shifting the bag will make as much difference in where the shots print on paper as almost anything else. Also, if parallax is not set correctly, you will shoot to different POI just by not having the exact same pressure on the stock or exact same eye position for each shot.
        The wind is not your friend.....unless you just farted.


        • #5
          I would suggest convincing yourself that a positional change will or will not change POI. Some rifles/calibers are not very sensitive to this while others (typically lighter, heavier recoiling) are very sensitive.
          Try shooting a group purposefully altering your body in a bad position after shooting a group well positioned behind the gun. You doing it and seeing the results will make it stick better and make you a better shooter.
          I agree with above about 3 shot groups - don't get too wrapped up in results from a couple 3 shot groups.


          • #6
            Upon free floating, how is it bedded? And how did you torque it? Maybe this is not stable.


            • #7
              I was not shooting three shot groups checking load development. I am aware of what the rifle is capable of. I said I was checking zero. You do not need 9 shots to check zero,
              in my opinion.
              The point of impact shift was actually three quarters.
              This is a factory rifle made in 1974. all I did was float the barrel. No bedding.
              I don't think the rifle shot left on its own. I think it was me not getting behind the rifle the same way by not shifting my hips to the right when transitioning to the target one foot left.


              • #8
                As the others have stated; not shifting your body with the rifle will for sure cause the odd change in point of impact. Natural point of aim is a very basic (read important) fundamental component of good marksmanship.

                If I go to zero my rifle and I shoot two shots that are very close to each other, use my reticle to measure the adjustment I need, dial the adjustment and fire two more shots that land exactly where they need to be my scope is still not zeroed correctly because I didn't shoot 7 shots? Not trying to derail the thread here.


                • #9
                  Originally posted by Skookum

                  Yes... you do need more than three shots to check zero. Three shots isn't enough to derive a true zero. To derive a true zero with any statistical significance you need at least seven shots, ten is better.

                  If you think different, then good luck to you.
                  the only time you need more than 3 rounds to check zero is if you or your gun doesnt shoot for dick


                  • #10
                    I would agree on that smaller sample size if you are stacking them. 5, at a good pace, will tell you a little more and 10 is even better. You are dealing with a system here that has a lot of variables. Shooter, rifle, wind, ammo, temperature, even lighting and contrast on the target and, for a FFP reticle, thickness of same. The more data points you can put into your computation of the cone of error, the more information you will have to judge what to do about that errant shot.
                    If you shoot for a living and spend all day at the range, have an OCD approach to reloading, know your powder and bullet performance and have a self cleaning rifle, maybe, just maybe, those 3 shots will give you all the information you need. I hardly think that describes most of us though.


                    • #11
                      Originally posted by Skookum

                      The first pair could count depending on how accurately you measured them. Even still, you would only have 4 shots. The more accurate your rifle/ammo combination is, the less error there will be in going with fewer shots.

                      But firing fewer shots, you won't know exactly how your rifle responds to barrel heat. Neither will you be able to accurately track performance and POI shifts of your chosen load from winter to summer. If hitting your target with the first shot in any conditions isn't the goal, then none of this matters. Accept the inherent error and adjust at will. You will probably be happier that way anyway.

                      True some data points can be gained by shooting at 100 yards and inaccurate rifles may require more shots, but doing this every time I zero is pointless. If fired 7-10 Shots every time I swapped barrels on my AT I'd waste a pile of ammo. I know my zero shift between barrels, I dial, verify with a few shots and head over to the long range to practice there. In your original post that I had referenced you mentioned only zeroing the rifle, this is a single data point. If you want to check how your barrel performs under heat and at different ambient temps then yes more shots would be better for this. However this would not need to be repeated every time the rifle is just zeroed. Furthermore, some of the mentioned data points would be better tested at range.

                      For example; My barrel had a 90 fps increase in muzzle velocity and at 100 yards this showed up as a slight zero shift, nothing more. I could have fired all day at 100 yards and not known that I had this increase. To me it could have been thought of as a barrel swap not returning to zero correctly, and just continued on. Now when I started shooting at 500 and 600 yards there was a drastic change in DOPE and this told me what was going on.
                      Same thing goes for how loads perform at different temps. Depending on the rifle a 50 fps increase may not show up in a 100 yard zero check but it will be noticeable at longer ranges.

                      To sum this up; I feel that for the level of precision of the OP's rifle and the fact that some data points are better tested at long range spending this much ammo just to zero is pointless.


                      • #12
                        Also double check scope parallax, could be an additional factor as well